feat(ui): weekend/vacation/checkbox colors follow accent theme

- Unify Saturday+Sunday into single isWeekend flag (header + grid lines)
- Replace hardcoded amber vacation bar/tooltip colors with brand-* classes
- Add global accent-color for checkboxes and radio buttons via CSS variable
- Update VACATION_TIMELINE_COLORS/BORDER to use brand palette (SICK stays red)
- Vacation-only tooltip uses neutral dark surface with brand accent border

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/app/globals.css

@@ -146,6 +146,11 @@
       linear-gradient(180deg, rgb(10 10 12 / 0.35), transparent 28rem);
   }
 
+  input[type="checkbox"],
+  input[type="radio"] {
+    accent-color: rgb(var(--accent-500));
+  }
+
   h1,
   h2,
   h3,